A speaker is an electro-mechanical transducer that reproduces an electrical signal as an acoustical signal. However, speakers are generally non-linear devices and consequently they introduce distortion when an electrical signal is to be reproduced.
U.S. Pat. No. 6,173,063 discloses a hearing instrument with a feedback configuration and a voltage regulator. The voltage regulator is provided to regulate voltage supplied by a battery supply to a class D output of the hearing instrument. In order to compensate for the undesired acoustical coupling from the speaker to the microphone of the hearing instrument, a feedback loop to cancel the effect of the undesired acoustical coupling is disclosed. The feedback loop extends from the output of a hearing instrument processor to the input of the hearing instrument processor.
US 2006/0188089 discloses methods and systems for echo cancellation in a speakerphone appliance connected to a telephone network. The speakerphone appliance has a station with a microphone and a loudspeaker, in addition to a handset with a loudspeaker and a microphone. A circuit is configured to measure the acoustical output from the loudspeaker of the station by means of the handset microphone. The measurement is used in a feedback system to reduce echo effects caused by the microphone and loudspeaker of the speakerphone appliance and reproduced in the acoustical output of the loudspeaker.
WO 96/26624 discloses audio system for a telephone with an adaptive pre-compensation filter for the correction of distortion in a loudspeaker. The pre-compensating filter models a non-linear speaker and receives an input signal representing a desired acoustic signal and provides an output signal for a loudspeaker via a loudspeaker drive unit. The pre-compensating filter is adaptively controlled via a filter modifier receiving the input signal and a signal from a microphone, which is adapted to pick up the acoustic signal produced by the loudspeaker. The pre-compensation filter is adaptively controlled so as to compensate for distortion produced by the loudspeaker.
However, the disclosed pre-compensation filter is not practical as a solution for a hearing instrument, since pre-compensation implies some insight in the actual non-linearity of a specific speaker. In the case of hearing instruments non-linearity may vary considerably from speaker to speaker in-situ in the ear canal of a hearing instrument user.